The continuously variable hydrostatic transmissions disclosed in the above-cited documents include a hydraulic pump unit and a hydraulic motor unit positioned in opposed, axially aligned relation with an intermediate, wedge-shaped swashplate. The pump unit is connected to an input shaft driven by a prime mover, while the motor unit is grounded to the stationary machine housing. An output shaft, coaxial with the input shaft and drivingly coupled to a load, is pivotally connected to the swashplate in torque-coupled relation. When the pump unit is driven by the prime mover, hydraulic fluid is pumped back and forth between the pump and motor units through ports in the swashplate. As a result, three torque components, all acting in the same direction, are exerted on the swashplate to produce output torque on the output shaft for driving the load. Two of these torque components are a mechanical component exerted on the swashplate by the rotating pump unit and a hydromechanical component exerted on the swashplate by the motor unit. The third component is a pure hydrostatic component resulting from the differential forces created by the fluid pressures acting on circumferentially opposed end surfaces of the swashplate ports, which are of different surface areas due to the wedge shape of the swashplate.
To change transmission ratio, the angular orientation of the swashplate relative to the axis of the output shaft is varied by a ratio controller. Since the transmission ratio, i.e., ratio of input shaft speed to output shaft speed, is continuously variable between 1:0 and 1:1, the prime mover can run at a constant speed set essentially at its most efficient operating point. The availability of a 1:0 (neutral) transmission ratio setting may eliminate the need for a clutch. As is indicated in cited U.S. Pat. No. 5,486,142, the swashplate can be positioned to angular orientations beyond the 1:0 ratio setting to provide limited infinitely variable speed drive in a reverse direction, as well as to angular orientations beyond the 1:1 setting to provide a limited, infinitely variable, overdrive speed range. Significantly, reverse drive is available without need for a reversing gear mechanism.
Unlike conventional, continuously variable hydrostatic transmissions, wherein hydraulic fluid flow rate increases proportionately with increasing transmission ratio such that maximum flow rate occurs at the highest transmission ratio setting, the flow rate in the transmissions disclosed in the cited applications reaches a maximum at a midpoint in the ratio range and then progressively decreases to essentially zero at the 1:1 transmission ratio setting. Thus, losses due to hydraulic fluid flow are reduced, and the annoying whine of conventional hydrostatic transmissions at high ratios is avoided. By virtue of the multiple torque components exerted on the swashplate, the decreasing hydraulic fluid flow in the upper half of the output speed range, and the capability of accommodating a prime mover input operating at or near its optimum performance point, the hydraulic machines of the cited U.S. patents and application have a particularly advantageous application as highly efficient, quiet, continuously variable hydrostatic transmissions in vehicular drive trains.
To accommodate operation at the infinitely variable transmission ratios set by the angular orientations of the swashplate, cylinder blocks of the hydraulic pump and motor units, pressed in sliding, interfacial engagement with opposed faces of the swashplate, are mounted by large spherical bearings concentric with the output shaft axis. These spherical bearings permit independent nutating motions of the pump and motor cylinder blocks during transmission operation; the amplitudes of these nutating motions are determined by the inclination angles of the engaging swashplate faces, which, in turn, are determined by the transmission ratio-setting angular orientation of the swashplate. It is the nutating motions of the pump and motor cylinder blocks that produce the hydraulic fluid pumping action of axially fixed pistons received in cylinders of the cylinder blocks. The swashplates disclosed in the cited patent documents consist of a unitary annular member having opposed faces, i.e., an input face in sliding contact with the pump cylinder block face and on output face in sliding contact with the motor cylinder block face. The input and output faces are formed to define an acute included angle of, for example, 12.degree., hence the wedge shape of the swashplate. This included angle is, however, fixed. Thus, transmission ratio is varied by changing the angular orientation of the swashplate, more particularly the angular orientations of the input and output faces of the swashplate. As disclosed in the cited documents, when the input face is oriented normal to the output shaft axis, the nutating magnitude of the pump cylinder block is zero. The nutating magnitude of the motor cylinder block is then necessarily high, essentially a maximum, neglecting the narrow reverse range. As a result, the transmission is set to a 1:0 ratio (neutral). Conversely, when the output face is normal to the output shaft axis, the nutating magnitude of the motor cylinder block goes to zero, and the nutating magnitude of the pump cylinder block is a maximum, neglecting the narrow overdrive range.
The fixed included angle between the swashplate input and output faces, however, imposes several limitations on the transmission. For example, when the wedge-shaped swashplate is oriented to a neutral setting, i.e., input face normal to output shaft axis, it is not a "true" neutral in the automotive sense. Because the motor cylinder block is nutating, the transmission output shaft is not free to turn, i.e., coast. Thus, in automotive applications, to achieve a "true" neutral at a swashplate neutral setting, a clutch must be included to decouple the transmission output shaft from the vehicle driving wheel(s) or to decouple the vehicle engine from the transmission input shaft. Alternatively, a "true" neutral may be achieved by depressurizing the hydraulic pump and motor units, as disclosed in Larkin U.S. patent application Ser. No. 08/543,545, filed Oct. 16, 1995.
Another limitation imposed by the fixed swashplate included angle is low end torque. It would be advantageous to elevate the motor cylinder block nutation beyond the maximum magnitude imposed by a fixed included angle as the swashplate is pivoted into the forward range from neutral and thereby increase output torque when it is most needed to accelerate a vehicle from a standing start. The fixed included angle also reduces the available ratio range in both overdrive and reverse.